Coding and decoding methods of a picture block, corresponding devices and data stream

ABSTRACT

A method for decoding a picture block is disclosed. The decoding method comprises:
         decoding decoded data, and information for identifying a reconstructed reference picture in a decoder picture buffer;   reconstructing another reference picture from the identified reconstructed reference picture identified by the information and from the decoded data;   replacing in the decoder picture buffer the reconstructed reference picture identified by the information with the another reference picture; and   reconstructing the picture block from the another reference picture.

This application claims the benefit, under 35 U.S.C. § 365 ofInternational Application PCT/EP2014/052998, filed Feb. 17, 2014, whichwas published in accordance with PCT Article 21(2) on Aug. 28, 2014 inEnglish and which claims the benefit of European patent applicationEP13305201.9, filed Feb. 22, 2013.

1. FIELD OF THE INVENTION

A method for decoding a picture block from a special reconstructedreference picture is disclosed. A corresponding coding method andcorresponding encoding and decoding devices are further disclosed.

2. BACKGROUND OF THE INVENTION

During video streaming, the bandwidth available may change over time.Consequently, the outgoing bit rate of the streaming application needsto be adjusted to fit the available bandwidth in real time in order toavoid congestion. One way to enable real-time bit rate adjustments isthe use of a real-time encoder, but it needs to allocate one encodingsystem per client that may be unacceptable in case of numerous clientsas for VOD services for example. Another way to enable real-time bitrate adjustments is the use of scalable video coding. In scalablecoding, a video source is encoded into several layers. During thetransmission in order to adjust the outgoing bit rate, the serverselects the layers to be sent (mode “push”) or the decoder asks for thelayers to be sent (mode “pull”). The method is suitable for streamingover heterogeneous channels, but scalable video coding degrades theoverall compression efficiency and increases the computationalcomplexity of both the encoder and the decoder compared to single layervideo coding. A simple method to realize bit rate adjustment is toencode multiple versions of the same video sequence. These versions havedifferent resolution and/or quality levels and thus different bit rates.During the streaming, when there is a need to adjust the outgoing bitrate, the stream to be transmitted can be switched dynamically from oneversion to the other in order to fit the bandwidth requirement or user'scapability as depicted on FIG. 1. This solution is known as “streamswitching”. However, directly switching between streams at inter-codedpictures (P or B pictures) may cause the mismatch of reconstructedreference pictures and results in incorrect pictures reconstruction. Thequality of reconstructed video may be degraded significantly. One methodto solve the problem is to use Random Access Points (RAP) in thebit-stream (typically I pictures or IDR pictures or CRA pictures). IDRis the English acronym of “Instantaneous Decoder Refresh” and CRA of“Clean Random Access”. As switching can take place at these RAP only,the RAP need to be assigned frequently in the bit stream in order torealize prompt stream switching. However, encoding such I/IDR picturesintroduce a substantial bit rate overhead. In addition, the picturesafter the RAP that uses reconstructed reference pictures located beforethe RAP are either skipped or not decoded correctly because they usereconstructed reference picture(s) which is/are different from theone(s) used in the encoding as depicted on FIG. 2. On FIG. 2, Ic isreconstructed from reconstructed reference picture I1 and I2 while itwas encoded from reconstructed reference picture i1 and i2.

In AVC, special picture types (SI/SP) were designed that allow foridentical reconstruction of a picture from another stream and thusfacilitate stream switching. Video pictures are thus encoded into SPpictures at switching points instead of intra-coded pictures as depictedon FIG. 3. The coding efficiency of the SP pictures is higher than thatof intra-coded pictures, but they are still less efficient than normal Ppictures. Therefore, the overall coding efficiency is still degraded ifmany switching points are assigned. In the document from Zhou et alentitled “Efficient bit stream switching of H.264 coded video” andpublished in proc. of SPIE vol. 5909 (2005), a solution is disclosedthat makes it possible to switch at any time without a substantial bitrate overhead. The solution is provided only for IPPP GOP structure. Inaddition to the multiple versions of the same video sequence atdifferent bit rate, a DIFF picture is encoded for the reconstructedreference picture of the current picture on which the switch occurs asdepicted on FIG. 4. The DIFF picture is the difference of thereconstructed reference picture of the current picture and the timelycorresponding picture in the other stream. The difference picture istransmitted to the decoder to compensate the mismatch. As the DIFFpicture is only transmitted when switching occurs as mentioned on page 5of the document, the bit rate overhead introduced by the above scheme issmall. On the other hand, the solution only works for P-picturepredicted from a single reconstructed reference picture. In addition,this solution requires that the encoding order and the display order areidentical.

3. BRIEF SUMMARY OF THE INVENTION

A method for decoding a picture block is disclosed. The decoding methodcomprises:

-   -   decoding at least one stream S_diff into decoded data and into        one information for identifying a reconstructed reference        picture in a decoder picture buffer;    -   reconstructing a special reference picture from at least the        identified reconstructed reference picture and from the decoded        data;    -   replacing in the decoder picture buffer one reconstructed        reference picture by the special reference picture; and    -   reconstructing the picture block from at least the special        reference picture.

According to a specific characteristic, the one reconstructed referencepicture replaced in the decoder picture buffer is the identifiedreconstructed reference picture.

Advantageously, the decoding method further comprises decoding aninformation representative of an instant in time at which the onereconstructed reference picture is replaced by the special referencepicture in the decoder picture buffer.

According to another aspect of the invention, the decoding methodfurther comprises decoding a flag indicating whether or not the specialreference picture is displayed.

According to a specific characteristic, the identified reconstructedreference picture is decoded from a base layer of a layered stream.

Advantageously, the decoded data and the information identifying thereconstructed reference picture in the decoder picture buffer aredecoded from an enhancement layer of the layered stream.

A method for encoding a picture block is also disclosed. The encodingmethod comprises:

-   -   encoding the picture block from at least one reconstructed        reference picture; and    -   encoding the at least one reconstructed reference picture as a        special reference picture from another reconstructed reference        picture and an information for identifying the another        reconstructed reference picture in a decoder picture buffer,        wherein the special reference picture when reconstructed        replaces one reconstructed reference picture in a decoder        picture buffer.

According to a specific characteristic, the one reconstructed referencepicture replaced in the decoder picture buffer is the identified anotherreconstructed reference picture.

Advantageously, the encoding method further comprises encoding aninformation representative of an instant in time at which the onereconstructed reference picture is replaced by the special referencepicture in the decoder picture buffer.

According to another aspect of the invention, the encoding methodfurther comprises encoding a flag indicating whether or not the specialreference picture is displayed.

According to a specific characteristic, the identified reconstructedreference picture is encoded in a base layer of a layered stream.

Advantageously, the at least one reconstructed reference picture and theinformation for identifying another reconstructed reference picture inthe decoder picture buffer are encoded in an enhancement layer of thelayered stream.

A decoding device for decoding a picture block is further disclosed. Thedecoding device comprises:

-   -   means for decoding at least one stream S_diff into decoded data        and into one information for identifying a reconstructed        reference picture in a decoder picture buffer;    -   means for reconstructing a special reference picture from at        least the identified reconstructed reference picture and from        the decoded data;    -   means for replacing in the decoder picture buffer one        reconstructed reference picture by the special reference        picture; and    -   means for reconstructing the picture block from at least the        special reference picture.

The decoding device is configured to execute the steps of the decodingmethod.

A coding device for encoding a picture block is disclosed. The codingdevice comprises:

-   -   means for encoding the picture block from at least one        reconstructed reference picture; and    -   means for encoding the at least one reconstructed reference        picture as a special reference picture from another        reconstructed reference picture and an information for        identifying the another reconstructed reference picture in a        decoder picture buffer, wherein the special reference picture        when reconstructed replaces one reconstructed reference picture        in a decoder picture buffer.

The coding device is configured to execute the steps of the encoding.

Finally, a data stream is disclosed that comprises encoded in it oneinformation for identifying a reconstructed reference picture in adecoder picture buffer and data allowing for the reconstruction of aspecial reference picture from the identified reconstructed referencepicture, the special reference picture being for replacing areconstructed reference picture in a decoder picture buffer.

4. BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will appear with thefollowing description of some of its embodiments, this description beingmade in connection with the drawings in which:

FIGS. 1 and 2 illustrate the general principles of stream switching;

FIG. 3 illustrates the principles of stream switching using SI/SPpictures according to the state of the art;

FIG. 4 illustrates the principles of stream switching using a DIFFpicture according to the state of the art;

FIG. 5 depicts the flowchart of a decoding method according to theinvention;

FIG. 6 depicts the flowchart of an encoding method according to theinvention;

FIG. 7 depicts a mono-layer video decoder according to the invention;

FIG. 8 depicts a mono-layer video encoder according to the invention;

FIG. 9 illustrates the principles of stream switching using SRP picturesaccording to the invention;

FIG. 10 illustrates a further embodiment of the decoding methodaccording to the invention;

FIG. 11 depicts a multi-layer video decoder according to the invention;

FIG. 12 depicts a multi-layer video encoder according to the invention;and

FIG. 13 represents a multi-layered stream according to the invention.

5. DETAILED DESCRIPTION OF THE INVENTION

The invention relates to a method for decoding a picture block of pixelsand a method for coding such a picture block. The picture block belongsto a picture of a sequence of pictures. Each picture comprises pixels orpicture points with each of which at least one item of picture data isassociated. An item of picture data is for example an item of luminancedata or an item of chrominance data. Hereafter, the coding and decodingmethods are described with reference to a picture block. It is clearthat these methods can be applied on several picture blocks of a pictureand on several pictures of a sequence with a view to the codingrespectively the decoding of one or more pictures. A picture block is aset of pixels of any form. It can be a square, a rectangle. But theinvention is not limited to such forms. In the following section theword block is used for picture block. In HEVC, the block refers to aCoding Unit (CU).

The “predictor” term designates data used to predict other data. Apredictor is used to predict a picture block. A predictor or predictionblock is obtained from one or several reconstructed reference sample(s)of the same picture as the picture to which belongs the block that itpredicts (spatial prediction or intra-picture prediction) or from one(mono-directional prediction) or several reference blocks(bi-directional prediction or bi-prediction) of reconstructed referencepictures (temporal prediction or inter-picture prediction). A referenceblock is identified in a reconstructed reference picture by a motionvector. The prediction can also be weighted to account for anillumination variation model (a.k.a weighted prediction).

The term “residue” signifies data obtained after subtraction of apredictor from source data.

The term “reconstruction” designates data (e.g. pixels, blocks) obtainedafter merging a residue with a predictor. The merging is generally a sumof a predictor with a residue. However, the merging is more general andnotably comprises an additional post filtering stage of reconstructedsamples and/or an additional step of addition of offsets to thereconstructed samples. When a reference picture is reconstructed, it isstored in the DPB (English acronym of “Decoder Picture Buffer”) as anewly reconstructed reference picture. In reference to the decoding ofpictures, the terms “reconstruction” and “decoding” are very often usedas synonyms. Hence, a “reconstructed block” is also designated under theterminology “decoded block”.

The term coding is to be taken in the widest sense. The coding possiblycomprises applying a transform and/or quantizing data. It can alsodesignate only the entropy coding. A DCT (“Discrete Cosine Transform) isan example of such a transform. In the same way, the term decodingpossibly comprises in addition to the entropy decoding, applying atransform and/or an inverse quantization. The transform applied on thedecoder side is an inverse transform of the one applied on the encoderside.

A stream is a sequence of bits that forms the representation of codedpictures and associated data forming one or more coded video sequences.Stream is a collective term used to refer either to a NAL unit stream ora byte stream.

A NAL (English acronym of “Network Abstraction Layer”) unit is a syntaxstructure containing an indication of the type of data to follow andbytes containing that data. The NAL is specified to format that data andprovide header information in a manner appropriate for conveyance on avariety of communication channels or storage media. All data arecontained in NAL units, each of which contains an integer number ofbytes. A NAL unit specifies a generic format for use in bothpacket-oriented and stream systems. The format of NAL units for bothpacket-oriented transport and byte stream is identical except that eachNAL unit can be preceded by a start code prefix and extra padding bytesin the byte stream format.

An AU (English acronym of “Access Unit”) is set of NAL units that areassociated with each other according to a specified classification rule,are consecutive in decoding order, and contain exactly one codedpicture. The decoding of an access unit always results in a decodedpicture.

In the FIGS. 5 and 6, the represented boxes are purely functionalentities, which do not necessarily correspond to physical separatedentities. As will be appreciated by one skilled in the art, aspects ofthe present principles can be embodied as a system, method or computerreadable medium. Accordingly, aspects of the present principles can takethe form of an entirely hardware embodiment, an entirely softwareembodiment (including firmware, resident software, micro-code, and soforth), or an embodiment combining software and hardware aspects thatcan all generally be referred to herein as a “circuit,” “module”, or“system.” Furthermore, aspects of the present principles can take theform of a computer readable storage medium. Any combination of one ormore computer readable storage medium(s) may be utilized.

The flowchart and/or block diagrams in the figures illustrate theconfiguration, operation and functionality of possible implementationsof systems, methods and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof code, which comprises one or more executable instructions forimplementing the specified logical function(s). It should also be notedthat, in some alternative implementations, the functions noted in theblock may occur out of the order noted in the figures. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, or blocks may be executed in an alternative order, depending uponthe functionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of theblocks in the block diagrams and/or flowchart illustration, can beimplemented by special purpose hardware-based systems that perform thespecified functions or acts, or combinations of special purpose hardwareand computer instructions. While not explicitly described, the presentembodiments may be employed in any combination or sub-combination.

-   -   FIG. 5 depicts the flowchart of a decoding method according to a        specific and non-limitative embodiment. The method is for        decoding a current picture block Bc encoded in a stream S. The        picture block Bc belongs to a slice Sc of a current picture Ic.        A slice is a part of a picture such as a set of picture blocks.

In a step 10, at least one stream S_diff is decoded into decoded data(e.g. residues and coding modes) and into an information INFO foridentifying a reconstructed reference picture R2 stored in a DPB.

In a step 12, a special reference picture (whose English acronym is SRP)R1′ is reconstructed from the identified reconstructed reference pictureR2 and from the decoded data. Reconstructing the SRP R1′ comprises, foreach picture block of R1′, determining a predictor and adding a residue.The predictor may be determined from the identified reconstructedreference picture R2 (either as a block in R2 co-located to Bc or as amotion compensated block in R2 thus identified by a motion vector) orfrom neighboring reconstructed samples as in classical intra prediction.A block in R2 is co-located to Bc if its spatial position in R2 isidentical to the spatial position of Bc in Ic. According to a variant,if the size of the reconstructed reference picture R2 is different fromthe size of the current picture Ic, then R2 is rescaled for thereconstruction of the special reference picture so that the rescaled R2picture (possibly with appropriate padding) has the same size as Ic. Inthis case, R1′ is reconstructed from F(R2), where F is a rescalingfilter. The stream S_diff may be a part of stream S or may beindependent of stream S.

As an example, the stream S_diff encodes the pixel by pixel differencebetween another reconstructed reference picture R1 different from R2 andthe reconstructed reference picture R2. R1 is for example thereconstructed reference picture from which the current picture block Bcis encoded. In this case, decoding the stream S_diff comprises decodinga difference picture DIFF usually by entropy decoding, inversequantization and transform. The transform is for example an inverse DCT.The difference picture is usually an approximation of the differencebetween the reconstructed reference picture R1 and the reconstructedreference picture R2. The approximation is due to the loss duringencoding (e.g. because of the quantization). If the difference pictureDIFF is lossless encoded, then the decoded difference picture DIFFequals the difference between the reconstructed reference picture R1 andthe reconstructed reference picture R2. According to a variant, if R1and R2 are of different sizes, the difference picture is the differencebetween the reconstructed reference picture R1 and the rescaledreconstructed reference picture R2. As an example, if R2 is larger thanR1 then R2 is downscaled and if R2 is smaller than R1 then R2 isup-scaled. In this case, the special reference picture R1′ equalsF(R2)+DIFF, F is the identity if R2 and Ic are of same size or F is arescaling function otherwise.

According to a variant, the decoding method further comprises anoptional decoding of a sign associated with the difference picture DIFF.If such a sign is decoded, the special reference picture R1′ equalsF(R2)+DIFF when the sign is positive and equals F(R2)-DIFF when the signis negative.

According to another variant, the stream S_diff encodes for some blocksof R1 the difference between these blocks and co-located blocks in R2.The other blocks of R1 are encoded in S_diff using classical intraprediction, i.e. from neighboring reconstructed samples.

According to another variant, the stream S_diff encodes for some blocksof R1 the difference between these blocks and corresponding blocks inR2. The corresponding blocks in R2 are either co-located blocks ormotion compensated blocks. The other blocks of R1 are encoded in S_diffusing classical intra prediction, i.e. from neighboring reconstructedsamples.

Decoding the information INFO makes it possible to handle different usecases. As an example, if the current picture block Bc is encoded fromtwo reconstructed reference pictures R1 and r1, then two specialreference pictures R1′ and r1′ and two information INFO and info aredecoded at step 10. The special reference pictures R1′ and r1′correspond respectively to R2 and r2, where R2 and r2 are tworeconstructed reference pictures stored in the DPB from which Bc is tobe reconstructed. Consequently, INFO indicates to the decoder that R1′is to be reconstructed from R2 while info indicates that r1′ is to bereconstructed from r2.

Each special picture is for example identified in the stream S_diff witha dedicated flag indicating a picture/slice type different from theclassical I, P, B picture/slice type. This picture/slice type indicatesthat the current AU contains a special reference picture that is to beused for replacing a picture in the DPB. According to a variant, eachspecial picture is identified with a dedicated flag in the slice header.

The information INFO for identifying in the DPB a reconstructedreference picture R2 is for example a POC (English acronym of “PictureOrder Count”) as defined in the document ISO/IEC 14496-10 (section3.104). According to a variant, the information for identifying areconstructed reference picture is a reconstructed reference pictureindex.

In a step 14, the special reference picture R1′ replaces one of thereconstructed reference picture in the DPB. According to a specificembodiment, the reconstructed reference picture in the DPB that isreplaced by the special reference picture R1′ is R2. The specialreference picture R1′ is thus stored in the DPB in the place of R2. IfBc is bi-predicted from two reference blocks belonging to tworeconstructed reference pictures R2 and r2 which may be different fromthe reconstructed reference pictures R1 and r1 used in the encoding,then both R2 and r2 are replaced in the DPB by R1′ and r1′. The specialreference pictures R1′ and r1′ are thus used as reference pictures forBc. Bc can also be reconstructed from one special reference pictures R1′and from r1, if r1 is available in the DPB when reconstructing Bc. In astep 16, the current picture block Bc is reconstructed from the specialreference picture R1′ instead of R2. Usually, since the specialreference picture is closer in terms of content to R1 than was R2, thedrift is thus decreased. If the replacement of step 14 is not made, thenBc is reconstructed from R2 instead of R1′. Usually, reconstructing apicture block comprises decoding a residue from the stream S and addingthe residue to a predictor. The residue can be zero in case of skipmode. Decoding the residue comprises entropy decoding, inversequantization and applying a transform inverse of the transform appliedon the encoder side. These steps are well known to those skilled in theart of video compression/coding and are not disclosed further. Areference block in the special reference picture R1′ is identified by amotion vector decoded from the stream S. The reference block is used asa predictor. In case of bi-prediction, two reference blocks areidentified in two reconstructed reference pictures which are possiblyone and the same reconstructed reference picture. The predictor is aweighted sum of these two reference blocks.

According to another variant, the decoding method further comprises anoptional step 13 wherein an information is decoded from the stream Srepresentative of a time instant Ts or to a decoding order numberindicating when the reconstructed reference picture R2 is replaced bythe special reference picture R1′. This data is useful in particularwhen the stream S_Diff is transported using separate means from the onesused to transport the stream S. This data is for example the POC of thefirst picture that uses this special reference picture in the DPB forits reconstruction. In this case, the reconstructed reference picture R2is replaced by the special reference picture just before starting thedecoding of this first picture. According to a variant, this data is forexample the POC of the last picture that uses the reconstructedreference picture R2 in the DPB for its reconstruction. In this case,the reconstructed reference picture R2 is replaced by the specialreference picture just after completion of the decoding of this lastpicture. According to yet another variant, a flag FG is decoded from thestream S_diff to indicate whether or not the special reference picturesare displayed. According to another variant, the flag is not decoded andthe SRP are by definition not displayed. INFO, sign, flag FG, timinginformation can be decoded for each special reference picture (in aslice header or in a slice segment header) or may be grouped for severalspecial reference pictures in one single header. INFO, sign, flag FG,timing information are for example decoded from a SEI message, VPS(Video Parameter Set HEVC) or from the slice header of Sc.

-   -   FIG. 6 depicts the flowchart of an encoding method according to        a specific and non-limitative embodiment. The method is for        encoding a current picture block Bc in a stream S.

In a step 20, a current picture block Bc is encoded from at least onefirst reconstructed reference picture R1 in a stream S. Usually,encoding the current picture block comprises determining a residue,transforming the residue and quantizing the transformed residue intoquantized data. The quantized data are further entropy coded in thestream S. The residue is obtained by subtracting from the currentpicture block Bc a predictor. The predictor is determined from the firstreconstructed reference picture R1. More precisely, a predictor isdetermined in the reconstructed reference picture R1 by a motion vector.If the current block is bi-predicted from two reference blocks, apredictor is obtained by averaging these two reference blocks. The tworeference blocks either belong to two different reconstructed referencepictures R1 and r1 or to one and the same reconstructed referencepicture. Motion vectors are also encoded in the stream S. These stepsare well known to those skilled in the art of video compression and arenot disclosed further. In a step 24, the reconstructed reference pictureR1 and an information INFO are encoded into the stream S_diff. Thedecoding of S_diff is a SRP. The stream S_diff may be a part of stream Sor may be independent of stream S. The reconstructed reference pictureR1 is encoded in S_diff from a second reconstructed reference picture R2different from R1 that is identified by INFO. According to a variant, ifthe size of the reconstructed reference picture R2 is different from thesize of the current picture Ic and thus from the size of R1, then R2 isrescaled for the encoding of the reconstructed reference picture R1 sothat the rescaled R2 picture (possibly with appropriate padding) has thesame size as Ic. In this case, R1 is encoded from F(R2), where F is anrescaling filter.

As an example, the stream S_diff encodes the pixel by pixel differenceDIFF between R1 and R2. The DIFF picture is encoded by transformation(e.g. using a DCT), quantization and entropy coding. According to avariant, if R1 and R2 are of different sizes, the difference picture isthe difference between the reconstructed reference picture R1 and therescaled second reconstructed reference picture R2. As an example, if R2is larger than R1 then R2 is downscaled and if R2 is smaller than R1then R2 is up-scaled. In this case, DIFF=R1−F(R2), F is the identityfunction when R2 and Ic are of the same size and is a rescaling functionotherwise.

According to a variant, the decoding method further comprises anoptional decoding of a sign associated with the difference picture. Ifsuch a sign is decoded, the special reference picture R1′ equalsF(R2)+DIFF when the sign is positive and equals F(R2)-DIFF when the signis negative.

According to another variant, the stream S_diff encodes for some blocksof R1 the difference between these blocks and blocks in R2 (i.e. eitherblocks co-located to Bc or motion compensated blocks). The other blocksof R1 are encoded in S_diff using classical intra prediction, i.e. fromneighboring reconstructed samples.

Encoding the information INFO makes it possible to handle different usecase. As an example, if the current picture block Bc is encoded from tworeconstructed reference pictures R1 and r1, then the two reconstructedreference pictures are encoded from two other reconstructed referencepictures R2 and r2. INFO indicates to a decoder that a special referencepicture R1′ is to be reconstructed from R2 while info indicates thatanother special reference picture r1′ is to be reconstructed from r2.Each special reference picture is for example identified in the streamS_diff with a dedicated flag indicating a picture/slice type differentfrom the classical I, P, B picture/slice type. This picture/slice typeindicates the current AU is a special reference picture that is to beused for replacing a picture in the DPB. According to a variant, eachspecial picture is identified with a dedicated flag in the slice header.In a specific embodiment, one special reference picture and aninformation INFO are encoded for several or each possible pairs ofreconstructed reference picture of the DPB. Consequently at any time ablock Bc can be decoded from any picture of the DPB even if it is notthe one from which it was encoded while limiting the drift. Indeed, whenreconstructing Bc, if R1 is not available in the DPB, Bc can bereconstructed from the special reference picture R1′ instead of R2. Thedrift is thus limited because R1′ is closer in terms of content to R1than is R2.

The information identifying a second reconstructed reference picture isfor example a POC. According to a variant, the information identifying asecond reconstructed reference picture is a reconstructed referencepicture index.

All the variants and options disclosed for the decoding method areapplicable to the encoding method. In particular, the encoding methodcomprises an optional encoding of a sign associated with the differencepicture. According to another variant, the encoding method furthercomprises an optional step 22 wherein an information is encoded that isrepresentative of a time instant Ts or to a decoding order numberindicating when the reconstructed reference picture R2 is to be replacedby the special reference picture in the DPB. This data is for examplethe POC of the first picture that uses this special reference picture inthe DPB for its reconstruction.

According to yet another variant, a flag FG is encoded to indicatewhether or not the special reference pictures are to be displayed. Thisflag can be coded for each special reference picture (in a slice headeror in a slice segment header) or may be grouped for several specialreference pictures in one single header. According to another variant,the flag is not encoded if by definition the SRP are not displayed.

INFO, sign, flag FG, timing information are for example decoded from aSEI message, VPS (Video Parameter Set HEVC) or from the slice header ofSc.

A decoder according to a specific and non-limitative embodiment isdepicted on FIG. 7. The decoder is adapted to implement the decodingmethod disclosed with respect to FIG. 5. A stream S is received by adecoding module DEC0. The decoding module DEC0 is adapted to decode aresidue for the current picture block Bc. The residue can be zero incase of skip mode. The decoding module DEC0 is connected to an adder ADDadapted to add a predictor to the residue. The predictor is determinedby a prediction module PRED from a reference picture previouslyreconstructed and stored in a DPB. The prediction module PRED determinesa predictor for the current picture block from at least one motionvector decoded by the decoding module DEC0. The DPB is connected to adecoding module DEC1 adapted to decode the stream S_diff into a decodeddata and into the information INFO. The decoding module DEC1 is furtheradapted to reconstruct special reference pictures from the decodedstream and from the reconstructed reference pictures stored in the DPBand identified by INFO. The special reference picture reconstructed bythe module DEC1 are used to replace in the DPB the picture identified byINFO. According to a variant not represented on FIG. 7, S_diff is a partof stream S.

An encoder according to a specific and non-limitative embodiment isdepicted on FIG. 8. The encoder is adapted to implement the encodingmethod disclosed with respect to FIG. 6. A subtraction module SUBTsubtracts a predictor from the current picture block Bc. The output ofthe subtraction module is a residue. The predictor is determined by aprediction module PRED from a reference picture previously reconstructedand stored in a DPB. The prediction module PRED determines a predictorfor the current picture block from at least one motion vector determinedby a motion estimation module ME. The residue is then coded by a codingmodule ENC0 into a stream S. The reference picture when encoded arefurther reconstructed by a module REC and stored in the DPB. The DPB isconnected to a module ENC1 adapted to encode in the stream S_diff thereconstructed reference picture R1 from another reconstructed referencepicture R2, e.g. by encoding a difference picture R1-R2, and theinformation INFO identifying R2. According to a variant not representedon the figure, S_diff is a part of the stream S.

According to a variant, the encoding and decoding methods are used inthe context of stream switching as illustrated by FIG. 9. In this case,a first sequence of pictures is encoded in a stream S0. A secondsequence of pictures is encoded in a stream S1. Usually, the secondsequence of pictures is identical to the first sequence but encoded at adifferent bit rate, i.e. by using different quantization step. Accordingto a variant, the second sequence of pictures is a rescaled version ofthe first sequence, i.e. either an up-scaled or a downscaled version.According to a specific embodiment, S0 and S1 have same GOP structure(i.e. same decoding order and same reference picture lists as defined insections 8.3.1 and 8.3.2 of the HEVC standard).

In addition to the streams S0 and S1, at each time instant to areconstructed reference picture R_(S1) ^(tn) of S1 is further encoded ina stream S_diff as a SRP from a timely corresponding, i.e. temporallyaligned, (e.g. identical picture order count or identical display time)reconstructed reference picture R_(S0) ^(tn) of S0 as depicted on FIG.9. The reconstructed reference picture R_(S1) ^(tn) is encoded in S_diffwith an information info_tn for identifying the correspondingreconstructed reference picture R_(S0) ^(tn). Note that the sourcepicture that corresponds to R_(S1) ^(tn) is encoded in S1 and the sourcepicture that corresponds to R_(S0) ^(tn) is encoded in S0.

The decoding method disclosed with respect to FIG. 5 is used fordecoding a picture block Bc after switching from the first stream S0 tothe second stream S1. With respect to FIG. 9, the pictures are decodedand displayed from the stream S0 until time t2. The switch occursbetween t2 and t3. After the switch the pictures are decoded anddisplayed from the stream S1. At the time of the switch the DBPcomprises several reconstructed reference pictures which where decodedfrom S0. With respect to FIG. 9, the DPB comprises three reconstructedreference pictures R_(S0) ⁰, R_(S0) ¹ and R_(S0) ² at the switchingtime.

In the step 10, S_diff1, S_diff2 and S_diff3 are decoded into decodeddata (e.g. residues and coding modes) and into information info_t0,info_t1 info_t2 identifying the reconstructed reference pictures R_(S0)⁰, R_(S0) ¹ and R_(S0) ² stored in the DPB.

In the step 12, three special reference pictures SRP_t0, SRP_t1, SRP_t2are reconstructed from corresponding decoded data and from correspondingreconstructed reference pictures R_(S0) ⁰, R_(S0) ¹ and R_(S0) ².According to a first specific embodiment, S_diff encodes the pixel bypixel difference between R_(S1) ^(tn) and the timely correspondingpicture R_(S0) ^(tn) possibly rescaled. In this case, the reconstructedSRP are SRP_t0=diff_t0+F(R_(S0) ⁰), SRP_t1=diff_t1+F(R_(S0) ¹),SRP_t2=diff_t2+F(R_(S0) ²), wherein diff_t0, diff_t1, diff_t2 aredecoded from S_diff. If necessary, R_(S0) ⁰ is rescaled by F so that itssize is the same as the size of the current picture Ic. If no rescalingoccurs then F is the identity function. According to a second specificembodiment, S_diff encodes R_(S1) ^(tn) using R_(S0) ^(tn) possiblyrescaled by F. In this case, the predictor of a block in R_(S1) ^(tn) iseither a spatially co-located block in the picture R_(S0) ^(tn) or amotion compensated block in R_(S0) ^(tn) or derived from spatiallyneighboring blocks in R_(S1) ^(tn) (spatial intra prediction). In thecase of the first specific embodiment, when no rescaling is necessary,i.e. when the sizes of the pictures of the first and second stream areidentical, then the same difference pictures diff_t0, diff_t1 anddiff_t2 can be used to switch from S0 to S1 or from S1 to S0. In theprevious example, if diff_t0 encodes the difference between R_(S0) ⁰ andthe timely corresponding picture R_(S1) ⁰ in the stream S1 instead ofthe inverse diff_t0 is subtracted from R_(S0) ⁰ instead of being addedin order to reconstruct SRP_t0. A sign is thus decoded to specify if thereconstructed reference pictures are modified by adding or bysubtracting the difference picture.

At step 16, the pictures in the DPB are replaced with the specialreference pictures. The information info_t0 indicates that SRP_t0 isused to replace R_(S0) ⁰ in the DPB, info_t1 indicates that SRP_t1 isused to replace R_(S0) ¹ in the DPB and info_t2 indicates that SRP_t2 isused to replace R_(S0) ² in the DPB. In this case, each SRP thusreplaces in the DPB the reconstructed reference picture from which it isreconstructed. The invention is clearly not limited to the case of 3reconstructed reference pictures. According to a specific embodiment ofthe invention, all reconstructed reference pictures in the DPB thatcomes from the stream S0 are replaced in step 16 with special referencepictures. According to a variant, only the reconstructed referencepictures in the DPB that are to be used as reference pictures after theswitch are replaced in step 16 with special reference pictures.

FIG. 10 illustrates a further specific and non-limitative embodiment ofthe decoding method according to the invention. The decoder receivesdifferent Access Units. The Access Unit AU1 is first received anddecoded. A first picture I1 is reconstructed from the decoded AU1. Then,a second Access Unit AU2 is received and decoded. A second picture I2 isreconstructed from the decoded AU2. The picture I1 and I2 belongs to thesame stream S0 and are stored in the DPB if they are signaled as used asreference pictures. Then, a switch occurs. The switch can be requestedby the decoder that sends a request to the encoder for receiving theS_diff stream. According to a variant, the switch is initiated by theencoder. Following the switch, the decoder receives two AU units S_diff1and S_diff2. S_diff1 and S_diff2 (step 10) are decoded in order toreconstruct (step 12) SRP1 and SRP2 using the picture I1 and I2respectively. SRP1 and SRP2 are two special reference pictures. SRP1 andSRP2 are then stored in the DPB in the place of the pictures I1 and I2respectively (step 14). Then the decoder receives AU3 and decodes it. Apicture I3 is reconstructed from the decoded AU3 and possibly from atleast one picture of the DPB (temporal prediction), i.e. either SRP1 orSRP2. I3 belongs to the second stream S1 and is possibly stored in theDPB for future use as a reconstructed reference picture. The decoderthen receives AU4 and decodes it. A picture I4 is reconstructed from thedecoded AU4 and possibly from at least one picture of the DPB (temporalprediction).

According to a specific embodiment of the invention, the pictures of thefirst and second sequences and the special reference pictures areencoded into a multi-layered stream. As a specific example, the pictureidentified as special reference pictures are encoded as an enhancementlayer of a scalable stream. S0 stream is a base layer. The enhancementlayer allows to reconstruct from reconstructed reference pictures of S0,special reference pictures to be used for reconstructing pictures of S1.This enhancement layer is for example compliant with SVC or SHVC codingstandard. According to a specific embodiment of the invention, thespecial reference pictures are encoded with a subset of the encodingtools/modes provided by SVC or SHVC for encoding enhancement layer. Inparticular, intra-layer motion vector prediction (temporal prediction)is disabled in SVC or SHVC coding standard. On the contrary, intraprediction from the base layer is activated. The intra pictureprediction may be activated too. According to another specificembodiment of the invention, the temporal my prediction is disabled forcoding S0 and S1 for example by setting the HEVC flagslice_temporal_mvp_enable_flag to false. This means that the motionvector prediction (MV prediction) is built using MV from reconstructedneighboring coding units, but not using the MVs of previouslyreconstructed reference pictures.

In the following FIGS. 11 and 12, encoding and decoding modules arereferred to as encoder and decoder.

FIG. 11 depicts a multi-layer encoder according to a specific andnon-limitative embodiment. The pictures of the first sequence areencoded in S0 using a first encoder ENC0 which is a mono-layer encoderfor example an MPEG2, an H.264 or an HEVC compliant encoder. Theinvention is not limited by the mono-layer encoder used. The referencepictures encoded with ENC0 are reconstructed as R2 and provided as inputto a third encoder ENC2 possibly after being rescaled by an optionalrescaling module F. The rescaled picture R2 is denoted F(R2). A secondencoder ENC1 is used to encode the pictures of the second sequence inS1. The second encoder can be identical to ENC0 or different. Theinvention is not limited by the encoder used. The reference picturesencoded with ENC1 that timely correspond to the reconstructed referencepictures R2 are reconstructed as R1 and provided as input to the thirdencoder ENC2. Therefore, for each reconstructed reference picture R2 inthe DPB of ENC0, a timely corresponding reference picture R1 isreconstructed. The encoder ENC2 thus encodes the reconstructed referencepictures R1 from the timely corresponding reconstructed referencepicture R2 possibly rescaled into the stream S_diff. According to aspecific embodiment the encoder ENC2 comprises a subtracter forsubtracting R2 respectively F(R2) from R1 and further an entropy coderfor encoding the difference picture thus obtained possibly transformedand quantized. According to a variant, from each block of R1 a predictoris subtracted, wherein the predictor is either a spatially co-locatedblock in the picture R2 (resp. F(R2)) or a motion compensated block inR2 (resp. F(R2)) or derived from spatially neighboring blocks in R1(spatial intra prediction). A residue is thus obtained and is furtherentropy coded after possibly being transformed and quantized. In thiscase, what is encoded in S_diff is not a simple pixel by pixeldifference between R1 and R2. An information INFO identifying thereconstructed reference picture R2 (resp. F(R2)) used to encode thereconstructed reference picture R1 is also encoded in S_diff. Theencoder ENC2 is for example compliant with a scalable video encoder suchas SVC or SHVC. The invention is not limited by the scalable encoderused. Scalable video codec standards define layer_id indicator toseparate/distinguish the AUs belonging to the Base Layer (BL) from theones belonging to the Enhancement Layers. According to a specificembodiment, the AU coming from ENC0 are encoded with a given layer_idwhich is different from the layer_id used to encode the AUs coming fromENC2. AUs coming from ENC0 and ENC1 have the same layer_id. ENC0 andENCC1 are for example single layer AVC or HEVC encoders. According to anadvantageous embodiment not represented on FIG. 11, the encoders ENC0and ENC1 are one and the same encoder. Therefore, pictures from thefirst and the second sequences are encoded with a same encoder andappropriate reconstructed reference pictures (R1, R2, F(R2)) areprovided as input to the second encoder ENC2.

FIG. 12 depicts a multi-layer decoder according to a specific andnon-limitative embodiment the invention. The first stream S0 is decodedusing a first decoder DEC0 which is a mono-layer decoder for example anMPEG2, an H.264 or an HEVC compliant decoder. The invention is notlimited by the mono-layer decoder used. The decoder DEC0 reconstructspictures from the first stream S0, in particular the reference picturesR2 which are stored in a DPB0. A second decoder DEC1 is used toreconstruct pictures from the second stream S1 which are stored in aDPB1. The second decoder can be identical to DEC0 or different. Theinvention is not limited by the decoder used. After a switch from S0 toS1, the reconstructed reference pictures R2 in DPB0 possibly rescaled byan optional rescaling module F, are copied into DPB1. The rescaledpicture R2 is denoted F(R2). A decoder DEC2 decodes (step 10) from thestream S_diff information INFO for identifying a reconstructed referencepicture R2 (resp. F(R2)) in the DPB1. The decoder DEC2 is for examplecompliant with a scalable video decoder such as SVC or SHVC. Theinvention is not limited by the scalable decoder used. The decoder DEC2further reconstructs (step 12) a special reference picture R1′ from thetemporally aligned reconstructed reference picture R2 (resp. F(R2)) andfrom data (e.g. residues, coding modes) decoded from S_diff. Accordingto a specific embodiment, the decoder DEC2 comprises an entropy decoderfor decoding a residue from S_diff and an adder for adding the residueto a predictor, wherein the predictor is derived either from co-locatedor motion-compensated blocks in R2 (resp. F(R2)) or from reconstructedsamples in R1′ (intra picture prediction). The special reference pictureR1′ is then stored (step 14) in the DPB1 in the place of R2 (resp.F(R2)) identified by INFO. DEC0 and DEC1 are for example single layerAVC or HEVC decoders. According to an advantageous embodiment notrepresented on FIG. 11, the decoders DEC0 and DEC1 are one and the samedecoder. Therefore, pictures are reconstructed from S0 and S1 with asame decoder DEC having a same DPB. Before a switch from S0 to S1,pictures are reconstructed from S0 by DEC and are possibly stored in theDPB. After the switch, appropriate reconstructed reference pictures (R2,F(R2)) are provided as input to the second decoder DEC2 forreconstructing SRP R′1. The reconstructed SRP R1′ are then stored in theDPB in the place of reconstructed reference pictures R2. Then, picturesare reconstructed from S1 by DEC using pictures stored in DPB (eitherclassical reconstructed reference pictures or SRP).

FIG. 13 represents a multi-layered stream according to a specific andnon-limitative embodiment the invention. On this figure the dashed linesrepresents the picture dependencies. AU1 and AU2 with layer_id=Layer_Aare received and decoded. Reference picture b1 and b2 are reconstructedfrom decoded AU and stored in the DPB. Upon switching, the AUs S_diff1and S_diff2 with layer_id=Layer_B are received and decoded. The decoderDEC2 then reconstructs special reference pictures e′1 and e′2 from datadecoded from S_diff1 and from S_diff2 and further from b1 and b2respectively. Information info_1 and info_2 decoded from S_diff1 andS_diff2 respectively are used to identify b1 and b2 respectively. Thespecial reference pictures e′1 and e′2 which are temporally aligned withb1 and b2 respectively are stored in the DPB in replacement ofreconstructed reference picture b1 and b2. Then, an AU3 is received anddecoded. A picture e3 is reconstructed from this decoded AU3 and furtherfrom the special reference pictures e′1 and e′2. The reconstructedpicture e3 is stored in the DPB since e3 is used as reconstructedreference picture for e4. An AU4 is received and decoded. A picture e4is reconstructed from the decoded AU4 and further from the specialreference picture e′2 and the reconstructed reference picture e3. Thefollowing AU5 and AU6 are received and decoded. Corresponding picturese5 and e6 are reconstructed from decoded AU5 and AU6. The DPB ispossibly updated if the reconstructed pictures are used as referencepictures. e′1 is preferentially an approximation of e1 one of thereconstructed reference pictures used when encoding e3. e′2 ispreferentially an approximation of e2 one of the reconstructed referencepictures used when encoding e3 and e4. The encoding and decoding methodsaccording to the invention makes it possible to realize flexible streamswitching while having a small bit rate overhead only when switchingoccurs. These methods are suitable for any GOP structure, any number ofreconstructed reference pictures and even when decoding order isdifferent form display order.

An example of a syntax is provided below within the SHVC coding standardframework for the S_diff stream.

Name of slice_type slice_type 0 B (B slice) 1 P (P slice) 2 I (I slice)3 SRP (SRP slice)

A slice_type is added to identify a slice of a special referencepicture.

De- slice_segment_header( ) { scriptor  first_slice_segment_in_pic_flagu(1)  ...  if( !dependent_slice_segment_flag ) {   for ( i = 0; i <num_extra_slice_header_bits; i++ )    slice_reserved_undetermined_flag[i ] u(1)   slice_type ue(v)   ... === Begin No IDR ===   if( !ldrPicFlag) {   ...   } === End No IDR ===   ... === Begin P or B ===   if(slice_type = = P || slice_type = = B ) {    ...   } === End P or B ====== Begin SRP ===   If (slice_type == SRP) { === i2 ===    sign_diff_picu(1) === i4 ===    pic_order_cnt_diffpic_apply_lsb u(v)   delta_poc_msb_diffpic_apply_present_flag[ i ] u(1)    if(delta_poc_msb_diffpic_apply_present_flag[ i ] )    delta_poc_msb_diffpic_apply_cycle_lt[ i ] ue(v) === i5===   no_output_diffpic _flag u(1) === i12===    num_layer_id_diffpic_applyu(6)   } === End SRP ===  ... }

sign_diff_pic equal to 1 indicates the residuals should be added to theprediction, else the residuals should be subtracted to the prediction.

pic_order_cnt_diffpic_lsb specifies the picture order count moduloMaxPicOrderCntLsb for this special reference picture. Then the intra BLprediction will use the reference picture in the DPB with samepic_order_cnt. This reconstructed special reference picture will replacethe reference picture in the DPB with same pic_order_cnt. The length ofthe pic_order_cnt_lsb syntax element is log2_max_pic_order_cnt_lsb_minus4+4 bits. The value of thepic_order_cnt_diffpic_lsb shall be in the range of 0 toMaxPicOrderCntLsb−1, inclusive. When pic_order_cnt_diffpic_lsb is notpresent, pic_order_cnt_diffpic_lsb is inferred to be equal to 0.

delta_poc_msb_diffpic_cycle_lt is used to determine the value of themost significant bits of the picture order count value of the long-termreconstructed reference picture in the DPB that this reconstructedspecial reference picture will replace in the DPB. Whendelta_poc_msb_cycle_lt is not present, it is inferred to be equal to 0

pic_order_cnt_diffpic_apply_lsb specifies the picture order count moduloMaxPicOrderCntLsb for the current picture. Then this special referencepicture will replace in the DPB the reference picture with POC equal topic_order_cnt_diffpic just before decoding the picture with POC equal topic_order_cnt_diffpic_apply_lsb. The length of the pic_order_cnt_lsbsyntax element is log 2_max_pic_order_cnt_lsb_minus4+4 bits. The valueof the pic_order_cnt_diffpic_lsb shall be in the range of 0 toMaxPicOrderCntLsb−1, inclusive. When pic_order_cnt_diffpic_lsb is notpresent, pic_order_cnt_diffpic_lsb is inferred to be equal to 0.

no_output_diffpic_flag equal to 1 indicates the picture to be replacedin the DPB should be displayed but not this special reference picture,else the this special reference picture should be displayed but not thereplaced reconstructed reference picture.

num_layer_id_diffpic_apply indicates the num_layer_id of thereconstructed reference pictures used to decode this special referencepicture.

Examples of Syntax (vps_Extension):

video_parameter_set_rbsp ( ) { Descriptor   ... diff_pic_flag_enabledU(1)   if ( diff_pic_flag_enabled ) {    no_output_diffpic _flag u(1)  inter_layer_pred_for_non_diff_pictures_flag u(1)  } }

diff_pic_flag_enabled equal to 1 indicates

no_output_diffpic_flag equal to 1 indicates the picture to be replacedin the DPB should be displayed but not this special reference picture,else the this special reference picture should be displayed but not thereplaced reference picture.

inter_layer_pred_for_non_diff_picture_flag equal to 1 indicates that anysubsequent picture of type I, P or B does not use inter-layerprediction, but pictures of type SRP may use inter layer prediction, butnot temporal intra layer prediction.

The video coders and decoders according to the invention and depicted onFIGS. 7, 8, 11 and 12, scalable or not, are for example implemented invarious forms of hardware, software, firmware, special purposeprocessors, or a combination thereof. Preferably, the present principlesmay be implemented as a combination of hardware and software. Moreover,the software is preferably implemented as an application programtangibly embodied on a program storage device. The application programmay be uploaded to, and executed by, a machine comprising any suitablearchitecture. Preferably, the machine is implemented on a computerplatform having hardware such as one or more central processing units(CPU), a random access memory (RAM), and input/output (I/O)interface(s). The computer platform also includes an operating systemand microinstruction code. The various processes and functions describedherein may either be part of the microinstruction code or part of theapplication program (or a combination thereof) that is executed via theoperating system. In addition, various other peripheral devices may beconnected to the computer platform such as an additional data storagedevice and a printing device.

According to variants, the coding and decoding devices according to theinvention are implemented according to a purely hardware realisation,for example in the form of a dedicated component (for example in an ASIC(Application Specific Integrated Circuit) or FPGA (Field-ProgrammableGate Array) or VLSI (Very Large Scale Integration) or of severalelectronic components integrated into a device or even in a form of amix of hardware elements and software elements.

The invention claimed is:
 1. A method comprising: decoding a referencepicture index identifying a reconstructed reference picture in a decoderpicture buffer comprising a plurality of reconstructed referencepictures, and further decoding data; reconstructing another referencepicture from said reconstructed reference picture identified by saidreference picture index and from said decoded data, said anotherreference picture and said reconstructed reference picture having a samepicture order count; replacing in said decoder picture buffer saidreconstructed reference picture identified by said reference pictureindex with said another reference picture; reconstructing a pictureblock from at least said another reference picture; and, decoding a flagindicating whether or not the another reference picture is displayed. 2.The method according to claim 1, wherein said reconstructed referencepicture identified by said reference picture index and said anotherreference picture are temporally aligned.
 3. The method according toclaim 1, further comprising decoding information representative of aninstant in time at which said reconstructed reference picture identifiedby said reference picture index is replaced by the another referencepicture in the decoder picture buffer.
 4. The method according to claim1, wherein the reconstructed reference picture identified by saidreference picture index is decoded from a base layer of a layeredstream.
 5. The method according to claim 4, wherein the decoded data andthe reference picture index identifying the reconstructed referencepicture are decoded from an enhancement layer of the layered stream. 6.A method comprising: encoding a reference picture from anotherreconstructed reference picture and a reference picture indexidentifying said another reconstructed reference picture in a decoderpicture buffer comprising a plurality of reconstructed referencepictures, said reference picture and said another reconstructedreference picture having a same picture order count, wherein said atleast one reference picture when reconstructed replaces said anotherreconstructed reference picture in the decoder picture buffer; encodinga picture block from said reference picture; and encoding a flagindicating whether or not the reference picture is displayed.
 7. Themethod according to claim 6, wherein said another reconstructedreference picture and said reference picture are temporally aligned. 8.The method according to claim 6, further comprising encoding informationrepresentative of an instant in time at which said another reconstructedreference picture is replaced by the reference picture in the decoderpicture buffer.
 9. The method according to claim 6, wherein the anotherreconstructed reference picture is encoded in a base layer of a layeredstream.
 10. The method according to claim 9, wherein said referencepicture and the reference picture index identifying the anotherreconstructed reference picture in the decoder picture buffer areencoded in an enhancement layer of the layered stream.
 11. A decodercomprising at least a processor configured to: decode a referencepicture index identifying a reconstructed reference picture in a decoderpicture buffer comprising a plurality of reconstructed referencepictures, and further decoding data; reconstruct another referencepicture from said reconstructed reference picture identified by saidreference picture index and from said decoded data, said anotherreference picture and said reconstructed reference picture having a samepicture order count; replace in said decoder picture buffer saidreconstructed reference picture identified by said reference pictureindex with said another reference picture; reconstruct a picture blockfrom at least said another reference picture; and decode a flagindicating whether or not the reference picture is displayed.
 12. Anencoder comprising at least a processor configured to: encode areference picture from another reconstructed reference picture and areference picture index identifying said another reconstructed referencepicture in a decoder picture buffer comprising a plurality ofreconstructed reference pictures, said reference picture and saidanother reconstructed reference picture having a same picture ordercount, wherein said at least one reference picture when reconstructedreplaces said another reconstructed reference picture in the decoderpicture buffer; encode a picture block from said reference picture; andencode a flag indicating whether or not the reference picture isdisplayed.
 13. The decoder according to claim 11, wherein saidreconstructed reference picture identified by said reference pictureindex and said another reference picture are temporally aligned.
 14. Thedecoder according to claim 11, wherein said at least one processor isfurther configured to decode information representative of an instant intime at which said reconstructed reference picture identified by saidreference picture index is replaced by the another reference picture inthe decoder picture buffer.
 15. The decoder according to claim 11,wherein the reconstructed reference picture identified by said referencepicture index is decoded from a base layer of a layered stream.
 16. Thedecoder according to claim 15, wherein the decoded data and thereference picture index identifying the reconstructed reference pictureare decoded from an enhancement layer of the layered stream.
 17. Theencoder according to claim 12, wherein said another reconstructedreference picture and said reference picture are temporally aligned. 18.The encoder according to claim 12, said at least one processor isfurther configured to encode information representative of an instant intime at which said another reconstructed reference picture is replacedby the reference picture in the decoder picture buffer.
 19. The encoderaccording to claim 12, wherein the another reconstructed referencepicture is encoded in a base layer of a layered stream.
 20. The encoderaccording to claim 19, wherein said reference picture and the referencepicture index identifying the another reconstructed reference picture inthe decoder picture buffer are encoded in an enhancement layer of thelayered stream.